The invention relates to repetition rate multiplication of pulsed light beams in general, and of mode locked lasers in particular. An important application of the invention is in the multiplication of the repetition rate of mode locked lasers for use in the production of printed circuit boards.
In applications such as printed circuit board (PCB) production, a laser may be used to expose a pattern on a photoresist coating on a copper coated substrate. In a typical exposure system such as the DP 100 of LIS of Germany, a CW UV laser beam is scanned across the PCB surface, while its intensity is modulated in accordance with a pattern to be generated. The modulating device receives electronic pixel data supplied by control circuitry. In modern PCB production it is desirable to operate at high data rates to increase production speed. The practical data rate is limited by the modulation rate and/or the available laser power.
In the production of PCBs utilizing UV sensitive photoresists an Argon Ion laser is often used. Although they are widely utilized as UV light sources, Argon lasers, being gaseous lasers, have a number of drawbacks such as complicate and delicate to operate, poor maintainability and/or high price.
Methods for producing UV laser radiation are known. For example, one such method utilizes an IR mode locked laser to generate high repetition rate laser light pulses. The frequency of the laser light is doubled to UV by passing the light through a non linear medium. However, the utilization of such lasers to photoresist exposure is severely limited by an inherent paradox: practical UV power drops rapidly with an increase in the repetition rate. This is because the frequency doubling process is extremely non linear and thus, its efficiency increases with increasing peak power. For a given average IR power, peak power drops as pulse repetition rate increases, leading to reduced UV generation efficiency.
U.S. Pat. No. 5,462,433 describes a device for use in electronic warfare for delaying coherent RF signals with an adjustable time delay element. The RF signals are divided into multiple signal paths, some of the paths are delayed relative to the other paths, and then the delayed and non delayed paths are recombined to increase the pulse repetition rate.
One broad aspect of some preferred embodiments of the present invention deals with the multiplication of the pulse repetition rate of optical signals. In preferred embodiments of the invention utilizing this aspect of the invention, the average power is substantially preserved. That is, the conversion process is practically lossless.
A second broad aspect of some preferred embodiments of the invention deals with the use of amplitude modulation of a pulsed light beam to transfer information and in particular to expose a photosensitive surface. In preferred embodiments of the invention, the pulse rate of the light pulses is at least several times the frequency of the modulation. Herein, such a combination of modulation of pulsed radiation is termed xe2x80x9cquasi-CWxe2x80x9d modulation. Thus, one aspect of some preferred embodiments of the invention deals with using quasi-CW modulated pulsed light to expose a resist on a PC board.
Quasi-CW modulation results when the pulse repetition rate is made high enough compared with the modulation data rate, so as to minimize the timing errors resulting from the lack of synchronization between the two. The pulse repetition frequency should be at least 2-3 times the pixel data rate to assure a smooth, error free written pattern.
A third broad aspect of the invention deals with the use of a pulsed laser, synchronized in time with the data modulation and having the same repetition rate in a PCB writing system. From a practical point of view however, the requirement that the pulse rate be exactly the same as the data rate and that the pulses be individually turned on and off make such a device more difficult to implement than the quasi-CW embodiments of the invention.
The present invention is generally described in the context of repetition rate multiplication of pulsed UV lasers used in direct writing on photoresists in PCBs production. However, as it will become evident, the present invention is applicable to the repetition rate multiplication of any pulsed light beam such as chopped, coherent and/or non coherent, monochromatic and/or non-monochromatic light beams, and/or stroboscopic light. To emphasize this broader applicability of the invention, the term xe2x80x9cpulsed light beamxe2x80x9d (PLB) is used herein to refer to light beams other than lasers and the terms xe2x80x9claser beamxe2x80x9d or xe2x80x9cpulsed laser beamxe2x80x9d is used when referring to pulsed lasers.
It is an object of some preferred embodiments of the present invention to provide a system and method for increasing the repetition rate of pulsed light beams. Preferably, the system will not substantially reduce the average power output of the light.
Another object of some preferred embodiments of the present invention, is to provide a laser system and a method for the use in direct writing on photoresists in the production of PCBs. Preferably, the system comprises a pulsed UV laser and a pulse rate multiplication device which multiplies the pulse repetition rate of the laser. In one aspect, some preferred embodiments of the present invention, allow for data rates higher than the laser""s pulse repetition rate prior to the multiplication.
In a preferred embodiment of the invention, a laser writing system, for example, for PCBs, is provided. This system utilizes a high power pulsed laser at a relatively long wavelength and low repetition rate, for example an IR mode locked laser operating at about 80 MHz and preferably having an average power of about 1 W. The system transforms this laser light to UV, preferably using a non linear optical medium. The pulsed UV light is amplitude modulated and used to scan and expose a PCB coated with a UV sensitive resist to form a pattern. Preferably, the UV light comprises a quasi-CW train of pulses such that methods known in the art may be used to modulate the UV light and utilize it to scan the area to be exposed.
It is an object of some preferred embodiments of the present invention, to provide a repetition rate multiplication apparatus and method for use in the multiplication of the repetition rate of pulsed light beams. Preferably, the repetition rate multiplication is obtained external to a generator that produces the pulsed light beam generator. In a preferred embodiment of the invention, the pulsed light beam generator is a pulsed laser. Preferably, the pulse rate conversion process is substantially lossless such that the average power in the light beam is preserved.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication method and apparatus which do not affect the PLB and/or laser beam generator initial functional specifications/characteristics such as efficiency, resonant cavity length, heat removal scheme, etc.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus which is retrofitted to an existing pulsed light source preferably, a pulsed laser.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus that is composed of passive components not requiring external power for their operation. Preferably, the passive components are optical components which do not require to be mechanically displaced in order for the apparatus to be operated.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus which will aim the beam pulses onto one target or simultaneously onto more than one target.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus and method which allows for variable delay of individual pulses, relative to each other to impose on the pulse train a predetermined configuration.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus and method which allow for the multiplication of the pulse repetition rate of a mode locked laser, preferably, without modifying the length of its resonant cavity.
There is thus provided, in accordance with a preferred embodiment of the invention, apparatus for producing high repetition rate optical pulses, including:
a beam generator that produces an initial pulsed light beam having an initial pulse repetition rate;
a pulse repetition rate multiplier, which receives the initial pulsed light beam and produces at least one pulsed light beam having a higher pulse repetition rate than the initial rate.
Preferably, the pulse repetition multiplier comprises:
a beam splitter, which receives the initial light beam and splits it into a plurality of split pulsed light beams; and
at least one optical delay path, which receives the split light beams and delays the split beams by different amounts,
wherein the pulsed delayed light beams taken together provide pulsed light having an increased pulse repetition rate which on the average is substantially equal to the initial rate multiplied by the plurality.
Preferably, the apparatus includes a beam combiner that receives the delayed beams and combines them into a single beam having the increased pulse repetition rate. Alternatively, the apparatus includes a beam combiner that directs the delayed beams to a given area such that the area is illuminated by light at the increased pulse repetition rate. Preferably, the beam combiner comprises a lens or more than one lens.
In a preferred embodiment of the invention, the optical delay path is comprises a beam delaying device, which includes at least one partially reflective mirror and at least one substantially fully reflective mirror.
Preferably, the optical delay path delays the split beams by an amount of time set by the distance between components of the beam delaying device.
In a preferred embodiment of the invention the apparatus includes:
a plurality of beam splitters; and
at least one substantially fully reflective mirror spaced from the beam splitters.
In a preferred embodiment of the invention at least one of the beam splitters is spaced at a different distance from the at least one substantially from the fully reflective mirror
Preferably, each of the beam splitters splits an incident beam into two equal portions.
In a preferred embodiment of the invention, where the plurality of beams equals N and wherein the plurality of beam splitters split an incident beam into a first beam having an intensity equal to 1/N, 1/(Nxe2x88x921), 1/(Nxe2x88x922) . . . 1/2 times the intensity of the incident beam and a second beam having an intensity of (Nxe2x88x921)/N, (Nxe2x88x922)/(Nxe2x88x921), (Nxe2x88x923)/Nxe2x88x922) . . . 1/2 times the intensity of the incident beam.
In a preferred embodiment of the invention, the optical delay path comprises a polarizing cube beamsplitter and two retro-reflectors.
In a preferred embodiment of the invention, the optical delay path comprises a polarizing cube beamsplitter and two retro-reflectors and wherein the beam combiner is comprised in the polarizing cube beamsplitter.
Preferably, the initial light beam is linearly polarized.
In a preferred embodiment of the invention the apparatus includes a second repetition rate multiplier that receives an output beam from the repetition rate multiplier and produces an output beam having a repetition rate higher than the repetition rate of the beam that it receives.
Preferably, the first repetition rate multiplier and the second multiplication rate multiplier each double the repetition rate. Preferably, the second repetition rate multiplier have the structure of the repetition rate multipliers defined above.
Preferably, the increased pulse repetition rate is twice, three times, four time or even
In a preferred embodiment of the invention the pulsed light beam generator generates a laser beam. Preferably, the laser beam generator comprises:
a pulsed laser operating at a first, relatively low, laser frequency;
a laser frequency doubler that doubles the laser frequency to produce the light beam.
Preferably, the pulsed laser comprises a mode locked laser. Preferably, the pulsed laser is an infra-red laser.
Preferably, the light beam is a UV laser beam.
In a preferred embodiment of the invention, the power contained in the higher repetition rate pulses is substantially equal to the initial pulsed light beam.
There is further provided, in accordance with a preferred embodiment of the invention, apparatus for transmitting information at a data rate comprising:
a pulsed light source that produces pulsed light having a pulse repetition rate, substantially higher than the data rate; and
a modulator what modulates the pulsed light at the data rate.
There is further provided, in accordance with a preferred embodiment of the invention, apparatus for recording an image on a photosensitive surface, comprising:
a pulsed light source that produces pulsed light having a pulse repetition rate;
a modulator that modulates the pulsed light at a data rate; and
a scanner that scans the modulated pulsed light over the surface.
Preferably, the pulsed light source is a line source and wherein the modulator spatially modulates the line.
Preferably, the photosensitive surface comprises a photoresist.
In a preferred embodiment of the invention, the data rate is substantially higher than the pulse repetition rate. Preferably, the pulse repetition rate is at least twice. three times, or four times the data rate. Alternatively, the data rate is the same as the pulse repetition rate.
Preferably, the pulsed light is laser light, preferably, comprised in a laser beam.
Preferably the pulsed light is produced in accordance with the apparatus described above.
There is further provided, in accordance with a preferred embodiment of the invention, a method for producing high repetition rate optical pulses, including:
providing an initial pulsed light beam having an initial pulse repetition rate;
multiplying the pulse rate of the initial pulsed light beam to produce at least one pulsed light beam having a higher pulse repetition rate than the initial rate.
Preferably, multiplying the pulse rate comprises:
splitting the initial light beam into a plurality of split pulsed light beams; and
delaying the split light beams by different amounts,
wherein the pulsed delayed light beams taken together provide pulsed light having an increased pulse repetition rate which on the average is substantially equal to the initial rate multiplied by the plurality.
Preferably the method includes combining the delayed beams into a single beam having the increased pulse repetition rate. Alternatively, the method includes combining the delayed beams and directing them to an area such that the area is illuminated by light at the increased pulse repetition rate.
In a preferred embodiment of the invention, the method includes further multiplying the pulse repetition rate of said at least one pulsed light beam having a greater rate to a an even higher repetition rate. In a preferred embodiment of the invention, multiplying and further multiplying each double the repetition rate.
Preferably, the increased pulse repetition rate is twice, three times, four times or more times the initial pulse rate.
Preferably, the pulsed light beam is a laser beam.
Preferably, the power contained in the higher repetition rate pulses is substantially equal to that contained in the initial pulsed light beam.
There is further provided, in accordance with a preferred embodiment of the invention, a method for transmitting information at a data rate comprising:
providing pulsed light that is pulsed at a pulse repetition rate, substantially higher than the data rate; and
modulating the pulsed light at the data rate.
There is further provided, in accordance with a preferred embodiment of the invention, a method for recording an image on a photosensitive surface, comprising:
providing pulsed light that is pulsed at a pulse repetition rate;
modulating the pulsed light at a data rate; and
scanning the modulated pulsed light over the surface.
Preferably, the pulsed light is a line of light and the modulator spatially modulates the line.
Preferably, the photosensitive surface comprises a photoresist.
In a preferred embodiment of the invention, the data rate is substantially higher than the pulse repetition rate. Preferably the pulse repetition rate is at least twice, three times or four times the data rate. Alternatively, the data rate is the same as the pulse repetition rate.
Preferably, the pulsed light is laser light, preferably, comprised in a laser beam. Preferably the pulsed light is produced in accordance with the methods described above.